Device for indicating the ceasing of super-conductivity



E. PERSSON 3,214,637

DEVICE FOR INDICATING THE GEASING OF SUPER-CONDUCTIVITY Filed April 4,1965 Fig 2 United States Patent 3,214,637 DEVICE FOR INDICATING THECEASING 0F SUPER-CONDUCTIVITY Erik Persson, Vasteras, Sweden, assignorto Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, acorporation of Sweden Filed Apr. 4, 1963, Ser. No. 270,698 Claimspriority, application Sweden, Apr. 9, 1962,

3 Claims. (Cl. 317-9) Certain materials have the property, at lowtemperatures in the neighbourhood of the absolute zero point, ofbecoming super-conducting, implying that the resistivity at a certaincritical temperature suddenly sinks to zero. The critical temperaturefor lead is about 7 K and for mercury 4 K. In recent times this propertyhas been utilised by using super-conductors in the magnetising windingof very large magnets with large field strength. Such magnets aresuitably built without an iron circuit. The necessary magnetising powerbecomes equal to zero since the super-conductor has no resistance, butof course a certain amount of power is consumed in keeping thesuper-conductor at the necessary temperature. As a rule the cooling isperformed by means of liquid helium.

Problems arise, however, if for some reason an increased temperatureoccurs at some point on the superconductor and the super-conductivity asa result ceases to exist at this point. If steps are not taken todisconnect the super-conductor quickly, it will melt at the fault point.The process becomes explosive since the very large magnetic energy ofthe magnet coil (of the order of 100 mws.) will be discharged in a shorttime in the nonsuper-conducting zone, which will be very short as theheat generated by current losses in the zone will not have time tospread to other parts of the coil.

The present invention relates to a protective device sensing thecessation of super-conductivity in a superconductor which encloses aflux generated by a current through the super-conductor. According tothe invention, a non-super-conducting'conductor is electricallyconnected to one end of the super-conductor to follow the superconductorclosely but is electrically insulated therefrom and between thenon-connected end points of the two conductors a voltage-sensing meansis connected, said voltage sensing means being so arranged that at apredetermined voltage diiference between the last-mentioned end pointsit initiates protective action by first connecting together the endpoints of the non-super-conducting conductor, possibly across a suitabledischarge resistor, and

thereafter breaking the super-conductor current circuit.

Since the super-conductor, when it is super-conducting, has noresistance and the other conductor carries no current and bothconductors are influenced by the same flux, the voltage differencenormally sensed by the voltagesensing means is zero. If thesuper-conductivity ceases at some point of the super-conductor, avoltage drop is obtained across the super-conductor which gives avoltage difference e between the non-connected end points of the twoconductors. The presence of this voltage difference e thus means that afault exists in the super-conductor and the voltage-sensing means isarranged to break the super-conductor current circuit.

It is important that the conductors follow each other as intimately aspossible so that due to a great many different leakage fluxes thevoltage e will not be generated on the occasion of faults other than afault in the superconductor. For this reason the non-super-conductor issuitably arranged to enclose wholly or partly the superconductor.

In spite of this arrangement of the two conductors it 'ice is impossibleto eliminate the circular flux between the conductors, which, on theoccasion of changes in the current through the super-conductor, mayinduce a voltage of the magnitude e across the voltage sensing means. Itis therefore advisable to provide the device with a delaying means,which is intended to obstruct the protective action when thevoltage-sensing means senses a predetermined voltage difference e ofonly short duration. The time delay chosen is so long that thevoltage-sensing means does not initiate action for the diiferentirrelevant disturbances which may occur, but not so long that thevoltage-sensing means cannot initiate action before a fault in thesuper-conductor causes its destruction.

The invention will now be described in greater detail, by way of examplewith reference to the accompanying drawings, in which FIGURE 1 is acircuit diagram of a device in accordance with the invention and FIGURES2 and 3 show how the non-super-conducting conductor may be arrangedaround the super-conductor.

In FIGURE 1, the numeral 1 indicates a super-conducting conductor and 2a non-super-conducting conductor which have been wound to form a coil.The conductors 1 and 2 are electrically connected at 4 and between theirends 5 and 6 a voltage-sensing means is connected. The coil is enclosedby a cooling system 7 filled with e.g. liquid helium. Thesuper-conductor is fed from a voltage source 8. The voltage-sensingmeans includes, inter alia, a time delay relay 10 and is operativelyconnected to a switching device in the form of a contactor 11. Thecontactor 11 receives its operating voltage from a voltage source 9. Thetime delay relay includes a coil 12 and a closing contact 13, and thecontactor consists of a coil 16, a breaking contact 17 and a closingcontact 18.

The device functions in the following manner. On the occasion of a faultin the super-conductor 1 a voltage difference e arises between thepoints 5 and 6. The contact 13 then closes with a certain delaydetermined by the dimensions of a capacitor 14 and a resistance 15. Whenthe contact 13 is closed a current flows through the coil, whereuponfirst the contact 18 is closed and thereafter the contact 17 is opened.This means that the end points of the non-super-conducting conductor 2are first connected across a resistor 19 and that the current circuit ofthe super-conductor is thereafter broken. The current in thesuper-conductor may be broken very quickly since it may be transferredto conductor 2. It may however be necessary to insert a damping resistor20 in order to take care of the little energy which remains in theleakage field between the conductors 1 and 2.

FIGURES 2 and 3, show possible ways of enclosing the super-conductor 1within the non-super-conducting conductor 2, there being a layer 3 ofelectrical insulation between the conductors. Having regard to the verylarge dynamic forces which act on a conductor in a magnet coil of themagnitude mentioned above, the embodiment according to FIGURE 3 isperhaps mechanically the more suitable.

I claim:

1. In a circuit including a super-conductor, means for protecting saidsuper-conductor in case of cessation of super-conductivity, saidsuper-conductor having a first and second end terminals and enclosing amagnetic flux generated by a current through said super-conductor, saiddevice including a non-super-conducting conductor arranged to followsaid super-conductor closely but electrically insulated therefrombetween said terminals, said non-super-conducting conductor having firstand second end points, said first terminal being electrically connectedto said first end point, a voltage sensing means connected between saidsecond end terminal and said second end point, a switching device insaid super-conductor circuit,

said voltage sensing means being operatively connected to said switchingdevice to initiate protective action in response to a predeterminedvoltage difference between said second terminal and said second endpoint, said switching device thereupon first connecting together saidend points of said non-super-conducting conductor and thereafterbreaking said current in said super-conductor.

2. In a circuit according to claim 1, said non-superconducting conductorbeing arranged to enclose said super-conductor at least partly. l

3. In a circuit according to claim 1, delaying means for preventingprotective action when said voltage sensing means senses a predeterminedvoltage difierence of short duration.

References Cited by the Examiner UNITED STATES PATENTS SAMUEL BERNSTEIN,Primary Examiner.

1. IN A CIRCUIT INCLUDING A SUPER-CONDUCTOR, MEANS FOR PROTECTING SAIDSUPER-CONDUCTOR IN CAE OF CESSATION OF SUPER-CONDUCTIVELY, SAIDSUPER-CONDUCTOR HAVING A FIRST AND SECOND END TERMINALS AND ENCLOSING AMAGNETIC FLUX GENERATED BY A CURRENT THROUGH SAID SUPER-CONDUCTOR, SAIDDEVICE INCLUDING A NON-SUPER-CONDUCTOR CONDUCTOR ARRANGED TO FOLLOW SAIDSUPER-CONDUCTOR CLOSELY BUT ELECTRICALLY INSULATED THEREFROM BETWEENSAID TERMINALS, SAID NON-SUPER-CONDUCTING CONDUCTOR HVING FIRST ANDSECOND END POINTS, SAID FIRST TERMINAL BEING ELECTRICALLY CONNECTED TOSAID FIRST END POINT, A VOLTAGE SENSING MEANS CONNECTED BETWEEN SAIDSECOND END TERMINAL AND SAID SECOND END POINT, A SWITCHING DEVICE INSAID SUPER-CONDUCTOR CIRCUIT, SAID VOLTAGE SENSING MEANS BEINGOPERATIVELY CONNECTED TO SAID SWITCHING DEVICE TO INITIATE PROTECTIVEACTION IN RESPONSE TO A PREDETERMINED VOLTAGE DIFFERENCE BETWEEN SAIDSECOND TERMINAL AND SAID SECOND END POINT, SAID SWITCHING DEVICETHEREUPON FIRST CONNECTING TOGETHER SAID END POINTS OF SAIDNON-SUPER-CONDUCTING CONDUCTOR AND THEREAFTER BREAKING SAID CURRENT INSAID SUPER-CONDUCTOR.